Blow molded plastic containers, such as pill bottles or beverage containers, are typically formed with recessed bottoms to provide increased structural stability. To produce such a recess, a “plug” is introduced in the bottom of the mold cavity so that as the parison stretches during the course of the blow cycle to meet the confines of the mold cavity, the bottom of the container forms around the protruding plug to conform to its shape. Subsequent to the blow cycle, in order to effect the removal of the container from the mold cavity, it is necessary to move the bottom plug away from the bottom of the container.
Typical injection blow molding machines have an upper and a lower die set. Typically, the lower die set is mounted to a machine bed, and the upper die set is configured to reciprocate vertically toward and away from the stationary lower die set. Mating blow mold halves are generally carried on the die sets to form closed blow mold cavities when the upper and lower die sets are pressed together. The cavities open when the upper die set is raised. A corresponding number of bottom plugs are arranged side-by-side and configured to align with the mold cavities.
It is known in the art to attach each bottom plug to a wedge, thereby creating a bottom plug assembly. Each wedge is provided with a pair of biased springs and corresponding guide pins. The biased springs of the prior art pushes the bottom plug assembly away from the mold cavity in a direction horizontal to the parting plane of the mold cavity and into a retracted position. The guide pins guide the movement of the bottom plug assembly as it enters into and retracts from the lower die set. The biased springs and guide pins are connected to the front of the wedge below the position of the bottom plug at one end, and to the lower die set at the other end. The lower die set typically includes holes for receiving the guide pins.
In order to push the bottom plug into the die set from its retracted position, an angled surface on the top rear surface of the wedge contacts a corresponding angled surface on the upper die set as the latter is lowered. The downward motion of the upper die set transmits a force through the angled surfaces and into the wedge. A portion of this force is directed horizontally, opposing the bias of the springs and pushing the bottom plug into the mold cavity. In this way each bottom plug is shifted horizontally into and out of its respective cavity as the upper die set is closed and then opened.
One problem with the prior art arises from the wear that occurs at the interface of the guide pins and the lower die set. That is, the upper die set exerts a vertical force at the top of the wedge, while the position of the guide pins is away from the top of the wedge, thereby creating a moment about the interface between the guide pins and the lower die set. As the guide pins resist the rotational force created by the moment, great stress is concentrated at the points of contact between the guide pins and the corresponding holes of the lower die set. After repeated cycles of advancing and retracting the bottom plug assembly, this stress will ultimately cause galling between the guide pins and the lower die set, thereby causing the pins to loosen with respect to the lower die set. The consequence is that the surfaces of the bottom plug and the mold cavity come into abrasive contact, ultimately destroying the integrity of one or both components. The present invention provides a bottom plug retract mechanism that overcomes the problems of the prior art.